Canning or otherwise packaging foods to preserve and store them for long periods of time has been an important part of food processing since the eighteenth century, when a Parisian chef named Appert devised a crude method of canning. Appert's process was introduced into the United States through England in about 1818. Canning remained an inexact process until Louis Pasteur applied his principles of fermentation to it in 1895.
Today, from picking to packaging, canning and bottling is a highly-developed, scientific industry. Foodstuffs are packaged in many different types of containers, with metal cans, glass jars and plastic packages being used on a wide scale. For convenience, the packaging of foodstuffs in rigid containers (i.e., cans, jars and rigid plastic packages) will be referred to collectively herein as “canning.” No matter what type of container is used, however, all canning processes must deal with the sensitivity of most foods to oxygen. As anyone who has sliced a fresh apple knows, oxygen in the air immediately begins to react with fresh foods and leads to the loss of their organoleptic qualities and to their rapid spoilage. All foods are sensitive to oxygen in varying degrees, and the successful preservation of foods by canning requires, as an important step, the elimination of oxygen from the containers.
Conventional canning processes rely on the use of liquids to displace oxygen and other gases from the containers. Typically, the foodstuffs being canned are placed in the containers and then covered with a liquid, which may be water, brine, or syrup. The covering liquid is preheated to a temperature of about 100° C. before it is added to the containers, or the containers are heated with liquid inside them. The liquid thus displaces the air and other gases in the containers. The containers are then sealed while at that temperature. Usually, the containers are also further heated to temperatures between 115° C. and 140° C. in order to sterilize the contents.
The lid or cap of the container may be placed loosely on the container before heating, so that it does not form a seal. The air within the container is then expelled through the remaining gap between the container and the lid. Alternatively, the heating may be carried out in a closed chamber from which air has been removed, in a steam atmosphere.
Flexible or partly flexible containers, for example, an open-topped rigid plastic container that has a flexible plastic film lid sealed across the open top, are used for various purposes. However, practical problems arise in using canning methods with such containers. In particular, the film lid is not sufficiently stable to be placed on the open top of the container without being sealed in place. If the lid is sealed at atmospheric pressure before sterilization, the high pressure inside the sealed container during sterilization can damage the seal. Because the seal is typically a thermoplastic weld, the seal may be weak at high temperatures. Some seals in general use can be damaged by an internal pressure of 0.1 atmosphere (about 10 kPa gauge pressure) at a temperature of 90° C. Conducting the entire canning process, including the sterilization, in a controlled steam environment to keep the internal and external pressures matched is inconvenient and expensive.
The process of the present invention embodies the advantages of canning without the concomitant disadvantages of prior processes, and results in a canned foodstuff which retains all its desirable organoleptic properties.